ABSTRACT: Formation of excessive sludge and fuel filter clogging were experienced in using biodiesel blends under marine environment. In this study, a field test was conducted in a marine ferry boat fueled by canola-based biodiesel blends. The sludge materials collected in the fuel purifier were characterized using inductively coupled plasma (ICP), pyrolysis-GC/MS (Py-GC/MS), thermogravimetric analysis (TGA), and Karl-Fischer titration. It was found that the sludge materials consisted of four distinctive fractions: organic materials derived from diesel and canola biodiesel (major fraction), ash (11-14% w/w), water (~17% w/w), and bacteria. The active bacteria were present in the sludge samples. It was suggested that bacterial contamination was one of the major factors in contribution to the sludge formation.